Nonionic surfactants, which are a safe class of enhancers, may offer means of enhancing drug permeation through the skin. In order to determine this effect, the influence of four nonionic surfactants on the percutaneous absorption of diclofenac sodium from carbopol gels containing 40% propylene glycol was investigated. In vitro diffusion experiments were carried out using excised full-thickness abdominal rat skin as well as cellulose nitrate membranes. The data of this study clearly revealed that Tween 80 decreased diclofenac penetration rate. This was due to a decrease in thermodynamic activity as a result of micellar complexation. In contrast, the more hydrophobic sorbitans enhanced diclofenac skin penetration, probably due to changes in the barrier properties of the skin and in the vehicle-stratum corneum partition coefficient. The most enhancing effect was induced by Span 20, a surfactant with a C12 saturated hydrophobic group. However, diffusional lag times for all the tested surfactants were longer than for the control gel.